Angiogenesis in fracture repair

Impaired angiogenesis and endochondral bone formation in mice lacking the vascular endothelial growth factor isoforms VEGF164 and VEGF188

Vascular endothelial growth factor (VEGF)-mediated angiogenesis is an important part of bone formation. To clarify the role of VEGF isoforms in endochondral bone formation, we examined long bone development in mice expressing exclusively the VEGF120 isoform (VEGF120/120 mice). Neonatal VEGF120/120 long bones showed a completely disturbed vascular pattern, concomitant with a 35% decrease in trabecular bone volume, reduced bone growth and a 34% enlargement of the hypertrophic chondrocyte zone of the growth plate. Surprisingly, embryonic hindlimbs at a stage preceding capillary invasion exhibited a delay in bone collar formation and hypertrophic cartilage calcification. Expression levels of marker genes of osteoblast and hypertrophic chondrocyte differentiation were significantly decreased in VEGF120/120 bones. Furthermore, inhibition of all VEGF isoforms in cultures of embryonic cartilaginous metatarsals, through the administration of a soluble receptor chimeric protein (mFlt-1/Fc), retarded the onset and progression of ossification, suggesting that osteoblast and/or hypertrophic chondrocyte development were impaired. The initial invasion by osteoclasts and endothelial cells into VEGF120/120 bones was retarded, associated with decreased expression of matrix metalloproteinase-9. Our findings indicate that expression of VEGF164 and/or VEGF188 is important for normal endochondral bone development, not only to mediate bone vascularization but also to allow normal differentiation of hypertrophic chondrocytes, osteoblasts, endothelial cells and osteoclasts.

Prevention of fracture healing in rats by an inhibitor of angiogenesis

Angiogenesis is considered essential to fracture healing, but its role in the healing process remains poorly understood. Angiogenesis inhibitors, which block new blood vessel formation by specifically targeting vascular cells, are currently under development for use in cancer chemotherapy, and are potentially powerful tools for defining the consequences of angiogenic impairment on fracture healing. In this study, we directly tested the effects of the angiogenesis inhibitor TNP-470 on the healing of closed femoral fractures in an established rat model system. Beginning 1 day after fracture, animals received either angiogenesis inhibitor at a therapeutically effective antitumor dose, or a weight-adjusted amount of carrier vehicle. The progress of fracture healing was assessed at weekly intervals for 21 days by radiography and histology; functional assessment was carried out at day 24 by biomechanical testing. By all three criteria, treatment with the angiogenesis inhibitor completely prevented fracture healing. Formation of both callus and periosteal woven bone were suppressed, indicating that both the intramembranous and endochondral pathways of osteogenesis were affected. The resulting tissue resembled "atrophic nonunions" often seen clinically in cases of failed fracture healing, but rarely achieved in animal models. These results show that angiogenesis is essential to very early stages of fracture healing, and suggest this model system may be useful for understanding the mechanisms underlying fracture nonunions due to vascular impairment. Finally, the data raise the possibility that impairment of fracture healing may be an adverse effect of clinical treatments with antiangiogenic drugs.

The angiogenic response to skeletal injury is preserved in the elderly

Angiogenesis is essential for normal bone formation and repair. Avascularity characterizes aberrant fracture union in the elderly, while angiogenic mechanisms during cutaneous wound repair are attenuated in aged humans. We hypothesized that skeletal injury results in local (circulating) and systemic (fracture site) 'angiogenic' responses and that these reparative mechanisms are attenuated with advanced patient age. This prospective study examined peripheral blood and fracture hematoma from 32 patients, 16 under 40 years and 16 over the age of 75, undergoing emergent surgery for isolated fracture. The angiogenic cytokines vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) were assayed. Endothelial cell cultures were supplemented with patient plasma and fracture hematoma and angiogenesis determined in vitro by measuring cell proliferation and blood vessel tube formation. Angiogenesis was determined in vivo using a murine dorsal wound pocket model and quantification of new blood vessel formation after 7 days. We found that all injured patients, irrespective of age, have elevated plasma and fracture hematoma levels of VEGF and PDGF. These elevated cytokine concentrations translate into biologically significant angiogenic effects, in vitro and in vivo. These effects are primarily VEGF mediated and are not dependent on patient age. The biological activity of these growth factors does not diminish with advanced age. Thus skeletal injury does result in local and systemic angiogenic responses whereby angiogenic cytokine availability and activity is preserved in the aged suggesting alternative mechanisms for the development of avascularity in delayed and fracture non-union in the elderly.

Rat mandibular distraction osteogenesis: latency, rate, and rhythm determine the adaptive response

Distraction osteogenesis is a well-established technique of endogenous tissue engineering. The biomechanical factors thought to affect the quality of the distraction regenerate include the latency, rate, rhythm, and consolidation period. In an effort to understand the impact of these parameters on regenerate bone formation, this study was designed to decipher the most adaptive response in a rat model of mandibular distraction osteogenesis. Ninety-six adult Sprague-Dawley rats were divided into 16 subgroups (n = 6 per subgroup) based on variations in the distraction parameters (i.e., latency, rate, and rhythm). After a 28-day consolidation period, the mandibles were harvested, decalcified, and sectioned. A standardized histologic ranking system was used to evaluate the effect of each protocol on the adaptive response of the regenerate bone. In this study, we have demonstrated that the latency period dramatically affects the success of distraction osteogenesis. Furthermore, distraction rates up to 0.50 mm per day stimulated excellent regenerate bone formation, whereas greater distraction rates produced a fibrous union. Finally, higher frequency distraction (i.e., increased rhythm) appeared to accelerate regenerate bone formation. We believe that defining the critical parameters of this model will improve future analysis of gene expression during rat mandibular distraction osteogenesis and may facilitate the development of biologically based strategies designed to enhance regenerate bone formation.

Osteoblast expression of vascular endothelial growth factor is modulated by the extracellular microenvironment

Angiogenesis, the formation of new blood vessels, is crucial to the process of fracture healing. Vascular disruption after osseous injury results in an acidic, hypoxic wound environment. We have previously shown that osteoblasts can produce vascular endothelial growth factor (VEGF) in response to a variety of stimuli. In this study we examined pH and lactate concentration, two components of the putative fracture extracellular microenvironment, and determined their relative contribution to regulation of rat calvarial osteoblast VEGF production under both normoxic and hypoxic conditions. Our results demonstrate that pH and lactate concentration do independently affect osteoblast VEGF mRNA and protein production. Acidic pH (7.0) significantly decreased VEGF production, under normoxic and hypoxic conditions (P < 0.05), compared with neutral pH (7.4). This decrease was primarily transcriptionally regulated, because the rate of VEGF mRNA degradation was unchanged at pH 7.0 vs. 7.4. Similarly, an elevated lactate concentration (22 mM) also depressed osteoblast elaboration of VEGF at both neutral and acidic pH (P < 0.001). Furthermore, the effects of increasing acidity and elevated lactate appeared to be additive.

Osteogenic protein-1, a bone morphogenetic protein, induces angiogenesis in the chick chorioallantoic membrane and synergizes with basic fibroblast growth factor and transforming growth factor-beta1

Capillary invasion is a vital regulatory signal during bone morphogenesis that is influenced by angiogenic molecules such as fibroblast growth factor (FGF) and some members of the transforming growth factor-beta (TGF-beta) superfamily, including TGF-betas themselves. Bone morphogenetic proteins (BMPs), which are members of the TGF-beta superfamily, have previously not been shown to possess direct angiogenic properties. Osteogenic protein-1 (OP-1; BMP-7) is a potent regulator of cartilage and bone differentiation in vivo. The osteogenic and angiogenic properties of OP-1 at both ortho- and heterotopic sites in adult chacma baboons (Papio ursinus) are enhanced synergistically by the simultaneous application of relatively low doses of TGF-beta1. The single application of relatively high doses of TGF-beta1 (20 ng), and bFGF (500 ng) or relatively low (100 ng) and high (1,000 ng) doses of OP-1 in the chick chorioallantoic membrane (CAM) assay elicited a prominent and (for OP-1) dose-dependent angiogenic response. The binary application of a relatively low dose of OP-1 (100 ng) with a relatively low dose of bFGF (100 ng) or with a relatively low (5 ng) or high (20 ng) dose of TGF-beta1 resulted in a synergistic enhancement of the angiogenic response. The angiogenic effect of the relatively low doses of the combined morphogens was distinctly more pronounced than that of the single application of the relatively high doses of the respective factors. The present findings suggest that these morphogens may be deployed in binary combination in order to accentuate experimental angiogenesis. The cooperative interaction of the different morphogens in the CAM assay may provide important biological clues towards the control of clinical angiogenesis.

VEGF expression in an osteoblast-like cell line is regulated by a hypoxia response mechanism

Angiogenesis is essential for the increased delivery of oxygen and nutrients required for the reparative processes of bone healing. Vascular endothelial growth factor (VEGF), a potent angiogenic growth factor, has been implicated in this process. We have previously shown that hypoxia specifically and potently regulates the expression of VEGF by osteoblasts. However, the molecular mechanisms governing this interaction remain unknown. In this study, we hypothesized that the hypoxic regulation of VEGF expression by osteoblasts occurs via an oxygen-sensing mechanism similar to the regulation of the erythropoietin gene (EPO). To test this hypothesis, we examined the kinetics of oxygen concentration on osteoblast VEGF expression. In addition, we analyzed the effects of nickel and cobalt on the expression of VEGF in osteoblastic cells because these metallic ions mimic hypoxia by binding to the heme portion of oxygen-sensing molecules. Our results indicated that hypoxia potently stimulates VEGF mRNA expression. In addition, we found that nickel and cobalt both stimulate VEGF gene expression in a similar time- and dose-dependent manner, suggesting the presence of a hemelike oxygen-sensing mechanism similar to that of the EPO gene. Moreover, actinomycin D, cycloheximide, dexamethasone, and mRNA stabilization studies collectively established that this regulation is predominantly transcriptional, does not require de novo protein synthesis, and is not likely mediated by the transcriptional activator AP-1. These studies demonstrate that hypoxia, nickel, and cobalt regulate VEGF expression in osteoblasts via a similar mechanism, implicating the involvement of a heme-containing oxygen-sensing molecule. This may represent an important mechanism of VEGF regulation leading to increased angiogenesis in the hypoxic microenvironment of healing bone.

Decreased nitric oxide levels stimulate osteoclastogenesis and bone resorption both in vitro and in vivo on the chick chorioallantoic membrane in association with neoangiogenesis

High nitric oxide (NO) levels inhibit osteoclast (OC)-mediated bone resorption in vivo and in vitro, and nitrate donors protect against estrogen-deficient bone loss in postmenopausal women. Conversely, decreased NO production potentiates OC bone resorption in vitro and is associated with in vivo bone loss in rats and humans. Previously, we reported that bone sections from rats administered aminoguanidine (AG), a selective inhibitor of NO production via inducible NO synthase, exhibited both increased OC resorptive activity as well as greater numbers of OC. Here, we investigated further whether AG promoted osteoclastogenesis, in addition to stimulating mature OC function, using a modified in vivo chick chorioallantoic membrane (CAM) system and an in vitro chick bone marrow OC-like cell developmental model. AG, focally administered in small agarose plugs placed directly adjacent to a bone chip implanted on the CAM, dose-dependently elicited neoangiogenesis while stimulating the number, size, and bone pit resorptive activity of individual OC ectopically formed in vivo. In addition to enhancing OC precursor recruitment via neoangiogenesis, AG also exerted other vascular-independent effects on osteoclastogenesis. Thus, AG promoted the in vitro fusion and formation from bone marrow precursor cells of larger OC-like cells that contained more nuclei per cell and exhibited multiple OC differentiation markers. AG stimulated development was inversely correlated with declining medium nitrite levels. In contrast, three different NO donors each dose-dependently inhibited in vitro OC-like cell development while raising medium nitrite levels. Therefore, NO sensitively regulates OC-mediated bone resorption through affecting OC recruitment (angiogenesis), formation (fusion and differentiation), and bone resorptive activity in vitro and in vivo. Possibly, the stimulation of neoangiogenesis and OC-mediated bone remodeling via AG or other pro-angiogenic agents may find clinical applications in reconstructive surgery, fracture repair, or the treatment of avascular necrosis.

Effects of ultrasound and 1,25-dihydroxyvitamin D3 on growth factor secretion in co-cultures of osteoblasts and endothelial cells

It has been shown that low-intensity pulsed ultrasound (US) accelerates fracture healing in animal models and in clinical studies. However, the mechanism by which US accelerates fracture healing remains unclear. Systemic factors and several growth factors, such as platelet-derived growth factor (PDGF), are thought to be involved in the process of fracture healing. In the present study, we examined the effects of US and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] on growth factor secretion in a co-culture system of human osteoblastic cells (SaOS-2) and endothelial cells (HUVEC). US was applied to cultured cells for 20 min daily for four consecutive days. US treatment increased the PDGF-AB level in the conditioned media. 1,25-(OH)2D3 (1 x 10(-8) M) also enhanced PDGF-AB secretion. The secretion of PDGF-AB was synergistically increased by the combination of US and 1,25-(OH)2D3. These results suggest that the stimulation of growth factor secretion from cells by US and 1,25-(OH)2D3 treatment may be involved in the acceleration of fracture healing.